hw/ac97: Make a bunch of mixer registers read only

The Linux ac97 driver tries to see if optional things like video input
volume control are available in 2 ways:
1) See if the mute bit is set after reset, if it is no further tests are done
2) If the mute bit is not set it does a write/read test of the mute bit

This patch changes our ac97 to conform to what the Linux driver expects, it
initializes registers for things which we don't emulate to 0 (so the mute bit
is not set) and makes them read only.

This causes Linux to now longer show the following (functionless)
controls in alsamixer:

Master Mono vol + mute
3d Control toggle
PCM out pre / post 3d select
Surround toggle
CD vol + mute
Mic vol + mute
Mic boost toggle
Mic mic1 / mic2 select
Video vol + mute
Phone vol + mute
Beep mono vol + mute
Aux vol + mute
Mono "output mic" / "mix" select
Sigmatel 4 speaker stereo toggle
Sigmatel ADC 6Db att toggle
Sigmatel DAC 6Db att toggle

This patch was also tested with a Windows XP guest and there it also makes
a number of functionless mixer controls go away.
Signed-off-by: NHans de Goede <hdegoede@redhat.com>
Signed-off-by: Nmalc <av1474@comtv.ru>

hw/ac97.c

@@ -54,6 +54,8 @@ enum {
     AC97_6Ch_Vol_C_LFE_Mute        = 0x36,
     AC97_6Ch_Vol_L_R_Surround_Mute = 0x38,
     AC97_Vendor_Reserved           = 0x58,
+    AC97_Sigmatel_Analog           = 0x6c, /* We emulate a Sigmatel codec */
+    AC97_Sigmatel_Dac2Invert       = 0x6e, /* We emulate a Sigmatel codec */
     AC97_Vendor_ID1                = 0x7c,
     AC97_Vendor_ID2                = 0x7e
 };
@@ -503,14 +505,16 @@ static void mixer_reset (AC97LinkState *s)
     memset (s->mixer_data, 0, sizeof (s->mixer_data));
     memset (active, 0, sizeof (active));
     mixer_store (s, AC97_Reset                   , 0x0000); /* 6940 */
-    mixer_store (s, AC97_Master_Volume_Mono_Mute , 0x8000);
+    mixer_store (s, AC97_Headphone_Volume_Mute   , 0x0000);
+    mixer_store (s, AC97_Master_Volume_Mono_Mute , 0x0000);
+    mixer_store (s, AC97_Master_Tone_RL,           0x0000);
     mixer_store (s, AC97_PC_BEEP_Volume_Mute     , 0x0000);
-
-    mixer_store (s, AC97_Phone_Volume_Mute       , 0x8008);
-    mixer_store (s, AC97_Mic_Volume_Mute         , 0x8008);
-    mixer_store (s, AC97_CD_Volume_Mute          , 0x8808);
-    mixer_store (s, AC97_Aux_Volume_Mute         , 0x8808);
-    mixer_store (s, AC97_Record_Gain_Mic_Mute    , 0x8000);
+    mixer_store (s, AC97_Phone_Volume_Mute       , 0x0000);
+    mixer_store (s, AC97_Mic_Volume_Mute         , 0x0000);
+    mixer_store (s, AC97_CD_Volume_Mute          , 0x0000);
+    mixer_store (s, AC97_Video_Volume_Mute       , 0x0000);
+    mixer_store (s, AC97_Aux_Volume_Mute         , 0x0000);
+    mixer_store (s, AC97_Record_Gain_Mic_Mute    , 0x0000);
     mixer_store (s, AC97_General_Purpose         , 0x0000);
     mixer_store (s, AC97_3D_Control              , 0x0000);
     mixer_store (s, AC97_Powerdown_Ctrl_Stat     , 0x000f);
@@ -657,6 +661,22 @@ static void nam_writew (void *opaque, uint32_t addr, uint32_t val)
                     val);
         }
         break;
+    case AC97_Headphone_Volume_Mute:
+    case AC97_Master_Volume_Mono_Mute:
+    case AC97_Master_Tone_RL:
+    case AC97_PC_BEEP_Volume_Mute:
+    case AC97_Phone_Volume_Mute:
+    case AC97_Mic_Volume_Mute:
+    case AC97_CD_Volume_Mute:
+    case AC97_Video_Volume_Mute:
+    case AC97_Aux_Volume_Mute:
+    case AC97_Record_Gain_Mic_Mute:
+    case AC97_General_Purpose:
+    case AC97_3D_Control:
+    case AC97_Sigmatel_Analog:
+    case AC97_Sigmatel_Dac2Invert:
+        /* None of the features in these regs are emulated, so they are RO */
+        break;
     default:
         dolog ("U nam writew %#x <- %#x\n", addr, val);
         mixer_store (s, index, val);